The present invention relates to automated scheduling and planning systems.
Resource planning is used extensively by industry. It is especially useful in the manufacturing sector, where careful scheduling of a manufacturing facility is necessary in order for such plants to be efficient. The flow of raw and partially finished goods, and scheduling of work on the various available machines, is a significant problem in large manufacturing facilities. A few examples of manufacturing facilities which are especially sensitive to scheduling problems include semiconductor fabrication facilities (front-ends), job shops, and plants making automobiles and heavy machinery.
The number of details and computations involved in completely scheduling a large manufacturing facility are enormous. No exact mathematical solution can, in general, be generated for such a facility. This is primarily because the facility does not operate in an ideal manner. Unforeseeable events are very common, including machine breakages, bad work which must be reworked or thrown away, and delays in moving material within the facility. These minute by minute events can have an impact on the overall operation of the facility and the precise nature of such impact cannot generally be determined in advance.
Many different schemes are currently in use for scheduling factory systems. These include the simplest scheduling system, that of no preplanned scheduling at all. In some factories, a work piece simply moves from machine to machine under the experienced guidance of the operator, and no particular pre-planning is made. In slightly more sophisticated systems, various rules of thumb are used by operators and process experts to control the flow of material through the plant. Some of these rules are very simple, such as FIFO (first-in-first-out). These rule of thumb decisions are made at a localized level. That is, the operator or expert will decide which workpiece should next go onto a particular machine based on the list of those workpieces currently available for the machine.
A more sophisticated system includes coordinated plant wide planning at some level. This is generally done by globally defining the manufacturing process and studying the interrelation between the various subprocesses therein. Such plant wide planning typically includes the identification of trouble spots such as bottlenecks in the overall process flow. An example of a state-of-the-art system would be OPT (Optimized Production Technology) which has been used for modeling and planning of manufacturing facilities since approximately 1979. The general theory of OPT is that plant capacity is determined by one or a small number of bottleneck processes. The overall strategy is then to ensure that the bottleneck processes are kept constantly busy by ensuring that queues are maintained in front of them. Desired work in process inventory levels at key points throughout the plant are determined at the global planning stage, and these desired values are compared to those which actually occur to determine the operating conditions within the plant.
Current sophisticated scheduling procedures generally begin with the creation of a global plan which outlines the overall characteristics of the manufacturing facility. Based on the current status of the facility, including such information as identification of work in process and machines which are down for repair, a general plan is made for some future time period. This plan will include directives such as "begin work on some number of identified items each hour for the next eight hours." Running a global plan periodically can be referred to as batch processing.
Batch processing of the global plan does not allow quick or easy response to changing conditions. If plant conditions change, such as a major piece of machinery going off-line for repair, the entire global plan must be recalculated. Such global plans do have the advantage that they take into account in the relationship between various parts of the manufacturing process, but they are relatively inflexible and can only be applied to broad concepts. Decision making at the level of a particular machine must still be done using rules of thumb.
Even in sophisticated systems, there is little interaction between the global plan and local decision making processes. The global plan cannot comprehend the effect of breakage of a particular machine in advance. Local decision making, that is, which work to load on which machine and in which order, is generally done by rules of thumb and cannot comprehend the effect of a particular action on overall plant operation. Planning is done only periodically at the global level, and often incorrect or inaccurate rules of thumb constitute the entire decision making process at a local level.
It would be desirable for a scheduling system to comprehend a global planning strategy combined with intelligent local decision making which considers the effect of local decisions elsewhere within the manufacturing process. It would be further desirable that such system be able to react to the numerous uncontrollable events which occur during the manufacturing process.
Therefore, a scheduling system includes a global, steady-state model of the entire manufacturing process. This global calculation is done one time and recalculated only when there is a major change in process flow definition or machine availability. This global plan generates parameters which are used to control local decision making strategies. The local strategies are applied to each machine in the manufacturing facility, and are relatively simple. Based upon the parameters extracted from the global definition, and information regarding the current state of the neighborhood of the particular machine, local decisions can be made on a real time basis. Special decision making strategies may be used by machines which are indentified as critical to the manufacturing process flow.
The novel features which characterize the present invention are defined by the appended claims. The foregoing and other objects and advantages of the present invention will hereafter appear, and for purposed of illustration, but not of limitation, a preferred embodiment is shown in the accompanying drawings.